And talking about consistency, as already pointed out by Jonathan, Cassandra actually provides a way to allow for strong consistency: also, with its tunable consistency levels, it provides greater freedom to implement different access patterns depending on the use case, as described on the blog post.
As someone has mentioned, Cassandra is able to be strongly consistent at the cost of a pretty significant performance cut. I am curious just how significant this performance cut would be to get consistency at the level of something like DynamoDB or HBase which is advertised as strongly consistent. Afterwards I'd be interested to see what the performance numbers would look like for all three.
I agree with you though, locking into Amazon is kind of a drawback :-(
First, in order to achieve strong consistency in a distributed system with replication enabled, you will always have to write to multiple replicas: provided the number of replicas is the same, whether you're using a consistent-hashing based replication (Cassandra) or a distributed file system (HBase) it really doesn't matter much, as I/O latencies (both network and disk related) will always play a large role here.
That said, you can have different performances if you:
1) Change the replication factor: this is something you can do regardless of the actual system you're using, so it doesn't count.
2) Change the number of "acknowledged" writes/reads as a fraction of the replication factor: strongly consistent systems will always provide at least as much acknowledges as Cassandra's QUORUM, so no big win here.
3) Change the point where you acknowledge writes, either memory, disk or physical sync: this is about durability, and if you use different settings for different systems (i.e. Mongo VS Cassandra) it is apples and oranges.
4) Change your physical storage structure (provided same level of durability as explained above): different databases use different structures, Cassandra uses Memtables+SSTables, which I don't expect to be slower than HBase HDFS-based ones.
So in the end, Cassandra is at least as fast as other distributed databases, even when working in strongly consistent fashion.
